Bipolar transistor, semiconductor device and method of manufacturing same

ABSTRACT

The bipolar transistor comprises a collector region ( 1 ) of a semiconductor material having a first doping type, a base region ( 2 ) of a semiconductor material having a second doping type, and an emitter region ( 3 ) having the first doping type. A junction is present between the emitter region ( 3 ) and the base region ( 2 ), and, viewed from the junction ( 4 ), a depletion region ( 5 ) extends into the emitter region ( 3 ). The emitter region ( 3 ) comprises a layer ( 6 ) of a first semiconductor material and a layer ( 7 ) of a second semiconductor material.  
     The first semiconductor material has a higher intrinsic carrier concentration than the second semiconductor material. The layer ( 7 ) of said second semiconductor material is positioned outside the depletion region ( 5 ). The second semiconductor material has such a doping concentration that Auger recombination occurs.  
     The invention also relates to a semiconductor device comprising such a bipolar transistor.  
     The method of manufacturing the bipolar transistor comprises the step of forming an emitter region ( 3 ) with a first doping type on a collector region ( 1 ) of a semiconductor material with a first doping type, and a base region ( 2 ) of a semiconductor material having a second doping type. The emitter region ( 3 ) is formed by epitaxially depositing a first layer ( 6 ) of a first semiconductor material and subsequently epitaxially depositing a second layer ( 7 ) of a second semiconductor material. The second layer ( 7 ) is doped with the first doping type, such that Auger recombination occurs. The intrinsic carrier concentration of the second semiconductor material is higher than the intrinsic carrier concentration of the first semiconductor material.  
     The Auger recombination dominates the base current and allows accurate tuning of the base current and the current gain of the bipolar transistor.

[0001] The invention relates to a bipolar transistor comprising

[0002] a collector region with a first doping type,

[0003] a base region with a second doping type,

[0004] and an emitter region with the first doping type,

[0005] a junction being situated between the emitter region and the baseregion, and, viewed from said junction, a depletion region extending inthe emitter region,

[0006] and, said emitter region comprising a layer of a firstsemiconductor material and a layer of a second semiconductor material.

[0007] The invention also relates to a method of manufacturing a bipolartransistor comprising a collector region with a first doping type and abase region with a second doping type, on which an emitter region withthe first doping type is formed, said emitter region including a layerof a first semiconductor material and a layer of a second semiconductormaterial.

[0008] U.S. Pat. No. 535,912 discloses a bipolar transistor that cansuitably operate at high frequencies. Said bipolar transistor has acutoff frequency of typically 100 GHz, as a result of which thetransistor can suitably be used as a component in optical communicationsnetworks for transporting 40 Gb/s.

[0009] The bipolar transistor is made from silicon and includes a baseregion with a Ge_(x)Si_(1−x) strained layer. As the bandgap ofGe_(x)Si_(1−x) is smaller than that of Si, with the conduction bandcoinciding with that of silicon, and the valence band energeticallymoved by ΔEv with respect to the valence band of Si, the charge storagein the base region and the emitter region is reduced relative to siliconbipolar transistors at comparable current levels. In order to maximizethe speed of the transistor, the percentage of Ge in the base region isas high as possible.

[0010] In the known bipolar transistor, the charge storage in theemitter is also reduced, which can be attributed to the fact that thebandgap, viewed from the junction, decreases linearly in the directionof the emitter contact. During operation of the bipolar transistor,minority charge carriers are injected into the emitter region from thebase region and accelerated by the internal electric field in theemitter, as a result of which the average residence time decreases.

[0011] The Ge_(x)Si_(1−x) strained layer in the base region causes achange of the bandgap ΔEv, as a result of which the collector currentincreases exponentially by ΔEv. As a result, the current gain, which isdefined as the quotient of the collector current and the base current,increases substantially. A drawback of a base region with Ge_(x)Si_(1−x)resides in that the current gain is too high, as a result of whichcollector-emitter breakdown occurs rapidly. The device is not robustbecause the bipolar transistor amplifies the current internally. Forpractical applications, a current gain of only approximately 100 isdesired.

[0012] In the known heterojunction bipolar transistor, the collectorcurrent is reduced by increasing the base doping. In addition, theemitter contact is made of a metal instead of the customarily usedpolysilicon. The recombination of minority charge carriers at a metalcontact exceeds that at a polysilicon contact by approximately one orderor magnitude, as a result of which the base current is increased byapproximately one order of magnitude.

[0013] A drawback of the known bipolar transistor resides in thatsetting the value of the base current is difficult. As the metal contactborders on the emitter region, and reacts at the interface with thesecond semiconductor material of the emitter region, the width of theemitter region, viewed from the junction, is highly subject tovariations.

[0014] As the width of the emitter region of a bipolar transistorintended for high-speed applications is very small, the decrease of theemitter width due to the interface reaction, causing a part of theemitter region to be consumed, is comparatively large. The base currentdepends very substantially on the width of the emitter region and theinterface between the emitter region and the metal. A metal contactleads to a substantial variation in base current between bipolartransistors and hence to a substantial variation in current gain.

[0015] It is an object of the invention to provide a bipolar transistorof the type described in the opening paragraph, which enables thecurrent amplification to be very accurately adjustable via the basecurrent.

[0016] As regards the bipolar transistor in accordance with theinvention, this object is achieved in that the intrinsic carrierconcentration of the second semiconductor material exceeds the intrinsiccarrier concentration of the first semiconductor material, the layer ofthe second semiconductor material is situated outside the depletionregion, and the second semiconductor material is doped such that Augerrecombination occurs.

[0017] When the bipolar transistor is in operation, minority chargecarriers injected from the base region into the emitter region diffusefrom the depletion region in the direction of an emitter contact thatborders on the emitter region. In the layer of the second semiconductormaterial, the intrinsic concentration n, of minority charge carriers isgreater than the intrinsic concentration in the first semiconductormaterial due to a smaller bandgap of the second semiconductor material.In a semiconductor, n_(i) ²=np, where n is the concentration ofelectrons and p is the concentration of holes, so that an increasedconcentration of minority charge carriers is present in the layer of thesecond semiconductor material. The physical effect causing an increasein base current is referred to as Auger recombination.

[0018] Auger recombination occurs if excess charge carriers recombine insemiconductor material having a high doping concentration. Theprobability of direct recombination between holes and electrons must notbe negligible relative to the recombination speed due to traps (SchottkyRead Hall recombination). In the case of Auger recombination, there arethree charge carriers that interact with each other, i.e. either twoelectrons and one hole, or two holes and one electron. Two chargecarriers recombine and the third charge carrier takes over the impulsefrom the incident charge carriers and the energy released by saidrecombination.

[0019] For an n-type emitter, the Auger recombination dependsquadratically on the electron concentration and linearly on the holeconcentration. Auger recombination contributes dominantly to the basecurrent if the hole concentration is increased by a number of orders ofmagnitude by the use of the second semiconductor material having asmaller bandgap and hence a higher intrinsic concentration. The increaseof the minority charge carriers depends exponentially on the decrease inbandgap. Thus, by accurately setting the bandgap as a function of thecomposition of the second semiconductor material, the base current canbe very accurately set, so that also the current amplification can bevery accurately set.

[0020] The first semiconductor material in the emitter region may be,for example, InAlAs, and the second semiconductor material may be, forexample, InGaAs. An N-type doping for these materials is, for example,silicon, and a p-type doping is, for example, beryllium. Alternatively,silicon may be used for the bipolar transistor comprising Si as thefirst semiconductor material and a Ge_(x)Si_(1−x) composition as thesecond semiconductor material. For the N-type doping use can be made,for example, of As or P, and for the p-type doping use can be made, forexample, of B.

[0021] Owing to the comparatively high intrinsic concentration, Ge canparticularly suitably be used as the second semiconductor material.

[0022] Advantageously, the second semiconductor material has acomposition that is at least substantially constant over at least a partof the layer. As a result, the bandgap is at least substantiallyconstant over said part as well as the intrinsic carrier concentration.In comparison with a situation where the composition of the secondsemiconductor material varies, a better setting of the Augerrecombination can be achieved in the part of the layer having the atleast substantially constant composition, so that the base current thatis dominated by Auger recombination can be more accurately set.

[0023] Preferably, the first semiconductor material of the emitterregion is silicon, and the second semiconductor material is acomposition of Si and Ge.

[0024] A great advantage of Ge_(x)Si_(1−x) resides in that, in terms ofenergy, the conduction band is at the same level as the conduction bandof silicon. By virtue thereof, it is possible not to influence thecollector current while the base current can be accurately adjusted bymeans of the percentage of Ge in the second semiconductor layer. As aresult of the smaller bandgap of the part of the layer with theGe_(x)Si_(1−x) composition, the hole concentration increases in thesemiconductor layer. Said increase of the hole concentration dependsexponentially on the decrease of the bandgap. The bandgap ofGe_(x)Si_(1−x) depends substantially linearly on the percentage of Ge.Auger recombination contributes dominantly to the base current if thehole concentration is increased by a number of orders of magnitude byusing Ge_(x)Si_(1−x).

[0025] An additional advantage is that the collector current and thespeed of the device, characterized by, inter alia, the cutoff frequencyf_(T), remains unchanged. As the current amplification can be reduced,the emitter-collector breakdown voltage BVceo increases and hence theproduct of f_(T)×BVceo increases too.

[0026] A further advantage resides in that the current amplification isless sensitive to temperature effects. Bipolar transistors carrying muchcurrent, such as power transistors, are internally heated by thecurrent, as a result of which the current amplification increases. As aresult of the smaller bandgap of Ge_(x)Si_(1−x) in the emitter region,the Ge_(x)Si_(1−x) in the emitter region has a negative temperatureeffect on the current amplification. This negative temperature effect atleast partly compensates for the positive temperature effect, as aresult of which the current amplification remains more constant as afunction of temperature.

[0027] For a high-speed bipolar transistor it is very important that thelifetime of the minority charge carriers is short. The lifetime τ of theminority charge carriers is approximately τ=1/(ΓN²), where in the caseof silicon Γ=2×10⁻³¹ cm⁶s⁻¹, and N is the doping in the part of thelayer of the emitter region. Thus, at a doping concentration of 3×10²⁰cm⁻³, the lifetime is typically 0.05 ns. Therefore, to obtain a shortlifetime, the doping concentration advantageously is as high aspossible, preferably above 3×10⁻²⁰ cm⁻³, in the part of the layerincluding the second semiconductor material.

[0028] It is advantageous that the part of the layer comprising thesecond semiconductor material is n-type doped. In general, npntransistors are faster than pnp transistors. The mobility for electronsis a few times higher than the mobility for holes, so that chargetransport of electrons is faster. In addition, the solubility of ann-type doping, particularly As, is much higher than that of a p-typedoping, such as B, so that comparatively many charge carriers areelectrically active.

[0029] In addition, n-type doping enables much shallower emitters to bemanufactured, so that charge storage in the emitter is comparativelysmall. In the manufacture of the transistor, the diffusion of n-typedoping atoms, such as As and Sb, takes place at a much lower rate thanthe diffusion of p-type doping atoms, such as B, so that much steeperdoping profiles are manufactured and the emitters become shallower.

[0030] The maximum percentage of germanium in the composition of thesecond semiconductor material is connected with the thickness of thelayer. As the lattice constant of germanium (5.66 Å) exceeds that ofsilicon (5.43 Å), compressive stress occurs in the Ge_(x)Si_(1−x) layerwhen this layer is epitaxially provided on a silicon lattice. If thestress in the Ge_(x)Si_(1−x) layer becomes too large, relaxation of thelayer takes place. If the percentage of Ge_(x)Si_(1−x) practicallyexceeds 30%, the Ge_(x)Si_(1−x) layer causes the stress to relax, sothat the layer is no longer properly epitaxial and lattice errors anddefects occur. Thus, in practice, the percentage of Ge remainscomparatively low.

[0031] It is important that the minority charge carriers cannot tunnelthrough the part of the layer, but instead Auger recombine in the secondsemiconductor material. For this reason, the part of the layer has awidth of at least several atomic layers, which, dependent upon thematerial, typically exceeds several nanometers. For a high Geconcentration of 30%, it is desirable, however, due to the stressrelaxation, that the layer is not too thick, i.e. the thickness shouldtypically be below 10 nm.

[0032] Preferably, the layer with the second semiconductor material atleast substantially adjoins the emitter contact. As, in general, thedoping is diffused in the emitter region by diffusion, the concentrationof doping atoms of the first type is highest at the surface. Here, Augerrecombination in the part of the layer of the second semiconductor isvery substantial. By virtue of the dominant effect of the Augerrecombination, the base current can be perfectly adjusted by varying theGe concentration. However, if the doping concentration in the emitterregion is at least substantially constant, it is advantageous if thelayer with the second semiconductor material at least substantiallyadjoins the depletion region, because, at said location, theconcentration of minority charge carriers is highest.

[0033] The bipolar transistor may be part of a semiconductor devicecomprising a semiconductor body of a first semiconductor material. Theinvention also relates to such a device.

[0034] The semiconductor device may be, for example, an integratedcircuit of the bipolar transistor and a CMOS circuit (BiCMOS) or amemory. The semiconductor body of the first semiconductor material maybe, for example, silicon, and the bipolar transistor may be aGe_(x)Si_(1−x) HBT.

[0035] Alternatively, the semiconductor body may be InP and the bipolartransistor may be an InAlAs/InGaAs HBT.

[0036] Another object of the invention is to provide a method ofmanufacturing the bipolar transistor of the type described in theopening paragraph, by means of which the value of the base current isaccurately defined.

[0037] As regards the method, the object of the invention is achieved,in accordance with the invention, in that a first layer of the firstsemiconductor material is epitaxially provided on the base region, afterwhich a second layer of the second semiconductor material issubsequently epitaxially provided and doped with a first doping type insuch a manner that Auger recombination occurs, and the intrinsic carrierconcentration of the second semiconductor material exceeds that of thefirst semiconductor material.

[0038] The first semiconductor material with the comparatively largerbandgap and the smaller intrinsic carrier concentration may be, forexample, InAlAs, and the second semiconductor material may be InGaAs.The layers are epitaxially grown on the base region, for example, bymeans of gas source molecular beam epitaxy. For the emitter use can bemade, for example, of a heavily doped n-type emitter, the doping beingprovided, for example, by means of ion implantation and diffusion. Thedoping concentration at which Auger recombination occurs depends on thesemiconductor material. The doping concentration is comparatively high,which generally leads to bandgap narrowing. The second semiconductormaterial may be, for example, a III-V semiconductor, germanium, or acompound of germanium, such as SiGe.

[0039] Advantageously, the composition of the second semiconductormaterial over the second layer is at least substantially constant. Aconstant composition of the material has the advantage that the bandgapis at least substantially constant and hence the intrinsic concentrationof charge carriers is also at least substantially constant. This enablesthe magnitude of the Auger recombination to be accurately adjusted.

[0040] An advantageous combination of semiconductor materials in theemitter region comprises Si as the first semiconductor material and acomposition of Si and Ge as the second semiconductor material. A greatadvantage resides in that the epitaxy cannot only be carried out using aslow growth method, such as MBE, but also by means of a fast depositionmethod, such as chemical vapor deposition. During the depositionprocess, the doping can be provided in situ. In this manner asubstantially constant doping level in the second semiconductor materialis guaranteed. As the whole emitter region has the same doping type, forexample the n-type, it is advantageous to n-type dope the firstsemiconductor material, so that in the growth process of the secondsemiconductor material with the same n-type doping, it is not necessaryto switch on the gases, as a result of which the doping is more uniformand autodoping does not occur.

[0041] The doping level that can be provided in situ in thesemiconductor materials depends on the temperature of the deposition andon the doping atom. The solubility of As is related to the temperatureat which deposition takes place. P has a lower solubility product andhence is less suitable for high doping levels in the emitter region. Sbhas a comparatively low solubility product, however, in the case ofclustering, a doping concentration of 1×10²⁰ cm⁻³ can be attained. Anadvantage of Sb resides in that the diffusion coefficient iscomparatively low, so that steep profiles can be obtained.

[0042] Alternatively, the high doping level in the emitter region can beprovided in that an emitter contact is formed on the emitter region byproviding a polysilicon layer having a doping of the first doping typeon the emitter region, and the second layer is doped throughoutdiffusion of the doping atoms of the polysilicon layer. The dopingmay have been provided in the polysilicon during the deposition process,however, in general the doping is provided in the polysilicon layerthrough ion implantation. Subsequently, during a step at a hightemperature of approximately 900° C., the doping atoms are diffused fromthe polysilicon layer into the emitter region. It is important to makesure that the diffusion time at high temperatures is short so as toobtain a shallow emitter region. To achieve this, use is often made ofrapid thermal annealing RTA, or laser annealing. The doping is broughtto high temperatures for a few seconds only, as a result of whichoutdiffusion is small.

[0043] In an advantageous method of manufacturing a semiconductor devicecomprising a semiconductor body of the first semiconductor material, thecollector and the base region are generally provided on a substrate. Inthe case of InAlAs/InGaAs transistors, the semiconductor material isInP.

[0044] The bipolar transistor of InAlAs/GaAs may be integrated with InPdevices so as to form an optoelectronic circuit that can very suitablybe used as a component in an optoelectronic network. A semiconductordevice comprising a semiconductor body of Si and a bipolar transistor ofsilicon can particularly suitably be used for BiCMOS and embeddedmemories.

[0045] The bipolar transistor is manufactured in a CMOS processrequiring only a few additional masking steps.

[0046] The emitter region can be selectively epitaxially grown in anemitter window of, for example, oxide and/or nitride, as described in WO9737377.

[0047] Growing the emitter in an emitter window by means of selectiveepitaxy is advantageous because it does not require additional maskingsteps. As selective growth is difficult, the emitter region mayalternatively be formed with a differential epitaxial layer, asdescribed in U.S. Pat. No. 5,821,149.

[0048] In general, it is advantageous to form the emitter at a latestage in the process, so that the thermal budget of the emitter issmall, as a result of which the emitter remains shallow and the dopingatoms are not electrically deactivated.

[0049] To form the emitter region in the semiconductor device, themethods described hereinabove apply, and all combinations also apply tothe semiconductor device.

[0050] These and other aspects of the invention will be apparent fromand elucidated with reference to the embodiment(s) describedhereinafter.

[0051] In the drawings:

[0052]FIG. 1 diagrammatically shows the bipolar transistor in accordancewith the invention;

[0053]FIG. 2 shows a first embodiment of the bipolar transistor inaccordance with the invention, wherein a part of the layer ofGe_(x)Si_(1−x) is situated in the emitter region, and the percentage ofGe has been varied.

[0054]FIG. 3 shows a Gummel plot of a bipolar transistor in accordancewith the first embodiment;

[0055]FIG. 4 is a diagrammatic, cross-sectional view of the bipolartransistor manufactured in accordance with the method;

[0056]FIG. 5 shows an experimental doping profile of the bipolartransistor in accordance with the first embodiment;

[0057]FIG. 6 shows an experimental Gummel plot of a bipolar transistorin accordance with the first embodiment, comprising 20% Ge in the partof the layer of the emitter region, and a reference without Ge.

[0058]FIG. 7 shows experimental data for the collector and base currentas a function of the percentage of Ge in the part of the layer of theemitter region;

[0059]FIG. 8 shows experimental data for the cutoff frequency as afunction of the percentage of Ge in the part of the layer of the emitterregion;

[0060]FIG. 9 shows experimental data for the collector-emitter breakdownvoltage of the bipolar transistor in accordance with the firstembodiment;

[0061]FIG. 10 is a diagrammatic, cross-sectional view of thesemiconductor device comprising the bipolar transistor manufactured inaccordance with the method.

[0062] The bipolar transistor shown in FIG. 1 comprises a collectorregion 1 with a first doping type, a base region 2 with a second dopingtype, and an emitter region 3 with a first doping type. A junction 4 ispresent between the emitter region 3 and the base region 2, and, viewedfrom said junction, a depletion region 5 extends in the emitter region3. The emitter region 3 comprises a layer 6 of a first semiconductormaterial and a layer 7 of a second semiconductor material.

[0063] The intrinsic carrier concentration of the second semiconductormaterial 7 is higher than that of the first semiconductor material 6.The second semiconductor material 7 is situated outside the depletionregion 5. The layer 7 comprising the second semiconductor material iscomparatively heavily doped. Said doping is such that Augerrecombination occurs. In the embodiment shown, the part 8 of layer 7,where the composition of the second semiconductor material is at leastsubstantially constant, is the whole layer 7.

[0064] In an advantageous embodiment of the bipolar transistor shown inFIG. 2, the layer 6 of the first semiconductor material is made ofsilicon and the layer 7 of the second semiconductor material is made ofa composition including silicon and germanium.

[0065] The layer 7 of SiGe is n-type doped with As, the profile beingsteep at a doping concentration above 3×10²⁰ cm⁻³. Said SiGe in theemitter region causes the equilibrium concentration of the holes to belocally increased. Auger recombination scales quadratically with then-type concentration and linearly with the hole concentration. Augerrecombination is used to increase the base current. The concentration ofholes increases by several orders of magnitude if the Ge percentage isincreased by, respectively, 10% and 20%. In the layer 7, the referencelevel of the hole concentration without Ge is indicated by means ofcurve a. The hole concentration at 10% Ge is indicated by means of curveb, and the hole concentration at 20% Ge is indicated by means of curvec. At 20% Ge, the hole concentration in the layer 7 increases by morethan one order of magnitude.

[0066] In this embodiment, the layer 7 has a thickness 9 of 10 nm. Thisthickness 9 of the layer is sufficient to cause Auger recombination ofsubstantially all holes. In this embodiment, the layer 7 is situated 5nm below the emitter contact 10. At the emitter contact 10, there isequilibrium concentration for holes. As both the Auger recombination andthe surface recombination at the emitter contact are linear with thehole concentration, it is obvious that the Auger recombination makes adominant contribution to the base current. After all, in the case of 20%Ge (curve c), the equilibrium concentration of holes at the emittercontact 10 is more than one order of magnitude smaller than in referencecurve a.

[0067] The layer 7 of the second semiconductor material 7 may border onthe emitter contact. As the interface between, for example, apolysilicon emitter contact and the monosilicon emitter region isgenerally not perfect, the part of the layer is situated just below thesurface of the emitter region and the emitter contact.

[0068] The effect of the increase of the Ge percentage on the basecurrent is visible in FIG. 3. At a collector-base voltage V_(CB) of 1 V,the base current increases by approximately a factor of 10 if thepercentage is increased from 0 to 20 percent. The initially excessivelyhigh current gain of 1300 is reduced to 130. The value of the collectorcurrent remains unchanged as the percentage of Ge increases. Also thecutoff frequency remains constant at 90 GHz.

[0069] The bipolar transistor may be part of a semiconductor devicecomprising a semiconductor body of a first semiconductor material. Thesemiconductor material of the bipolar transistor may be crystallinesilicon, III-V semiconductors, Si—Ge, Si—C layers, or other compounds.

[0070] In FIG. 4, parts corresponding to parts of FIG. 1 are indicatedby means of the same reference numeral. In the method in accordance withthe invention, an emitter region 3 with a first doping type is formed ona collector region 1 with a first doping type and a base region 2 with asecond doping type. The emitter region 3 is formed by epitaxiallyproviding a first layer 6 of the first semiconductor material, afterwhich a second layer 7 of the second semiconductor material issubsequently epitaxially provided and doped with the first doping typein such a manner that Auger recombination occurs. The intrinsic carrierconcentration of the second semiconductor material exceeds that of thefirst semiconductor material.

[0071] In an advantageous method in accordance with the invention, a 0.4μm epi layer with a P-doping of 5×10¹⁷ cm⁻³ is present on a heavilydoped n-type substrate. There is started from a base region 2 of, forexample, a differentially, epitaxially grown layer packet of 20 nmintrinsic Ge_(x)Si_(1−x) (x=0.18), 5 nm Ge_(x)Si_(1−x) (x=0.18) dopedwith 6×10¹⁹ cm⁻³ boron and 10 nm intrinsic Ge_(x)Si_(1−x) (x=0.18).

[0072] The emitter region is formed on the base region. A 100 nm thicklayer of silicon is epitaxially grown on the base region. This firstlayer 6 is doped with, for example, 3×10¹⁸ cm⁻³ phosphor. After 85 nm, asecond layer of 10 nm Ge_(x)Si_(1−x) (x=0.2) is epitaxially grown. Thecomposition of the second semiconductor material on the second layer isat least substantially constant. In this embodiment, this secondsemiconductor layer 7 of Ge_(x)Si_(1−x) is also doped with phosphor, thedoping level being 3×10¹⁸ cm⁻³.

[0073] The doping profile of the transistor after the epitaxial growthof the emitter region, shown in FIG. 5, exhibits a high Ge peak justbelow the emitter surface. The Ge concentration decreases substantiallyas a function of depth, viewed from the emitter surface. It ispractically impossible to grow a box-shaped profile. Only acomparatively small part 8 of the layer 7 has a substantially constantSiGe composition. This does not impose any limitations on the Augerrecombination process.

[0074] A layer of polysilicon 16 is deposited on the emitter region 3.In the embodiment shown in FIG. 4, the polysilicon is deposited in awindow of an isolating material. The isolating material is oxide 17 andnitride 18. The polysilicon layer can be doped by means of ionimplantation or it can be doped in situ. In this embodiment, thepolysilicon is doped in situ with 3×10²⁰ cm⁻³ phosphor atoms. The n-typedoping atoms are diffused from the polysilicon into the emitter regionduring 10 seconds at a temperature of 985° C.

[0075] Subsequently, the polysilicon emitter contact is patterned bymeans of lithography and etching, and finally connected to a metal 19.

[0076] In the Gummel plot shown in FIG. 6, the emitter region 3 has asurface area of 0.3×10 μm². At a collector-base voltage V_(CB) of 0 V,the base current demonstrates a substantial increase at a Ge percentageof 20% in the emitter region (curve a) with respect to the base currentwithout Ge in the emitter region (curve b). The value of the collectorcurrent at 20% Ge in the emitter region is substantially identical tothe value of the collector current without Ge in the emitter region. Theinitial current gain, which is above 1000, is reduced by more than oneorder of magnitude owing to the increased Auger recombination

[0077] It is shown in FIG. 7 that, at an emitter-base voltage of 0.7 V,the collector current does not depend on the Ge percentage, whereas thebase current increases substantially at an increased Ge percentage inthe emitter region.

[0078] The high-frequency behavior is not influenced by the Augerrecombination. FIG. 8 shows that the cutoff frequency does not depend onthe percentage of Ge in the emitter region, and it has a value oftypically 40 GHz for this transistor. However, it is very well possibleto achieve higher cutoff frequencies. The transistor comprising SiGe inthe base region in accordance with the first embodiment has a cutofffrequency of 90 GHz.

[0079]FIG. 9 shows that the breakdown voltage of the emitter-collectorBV_(CEO) has increased substantially by the addition of 20% Ge in thepart of the layer of the emitter region. The f_(T)×BV_(CEO) product isindicative of the speed of a bipolar transistor. As the cutoff frequencyis not influenced by the addition of Ge in the part of the layer of theemitter region, the f_(T)×BV_(CEO) product has increased by 25% andhence also the speed of the transistor at the same BV_(CEO).

[0080]FIG. 10 shows a semiconductor device 11 comprising a semiconductorbody 12 of a first semiconductor material, provided with the bipolartransistor manufactured in accordance with the invention. The bipolartransistor is integrated with CMOS in a so-termed BiCMOS process. Thebipolar transistor comprises SiGe as the second layer of semiconductormaterial in the emitter region 3. As the Ge_(x)Si_(1−x) strained layeris metastable, it is undesirable to allow the temperature to rise above900° C. for a prolonged period of time during the manufacturing process.For this reason, generally first the CMOS devices and, for example,embedded non-volatile memories are manufactured, while the emitterregion 3 with the second layer 7 of Ge_(x)Si_(1−x) are formed in a laststep.

[0081] It is to be noted that the invention is not limited to theexamples described hereinabove, and that the invention can be employedin each bipolar transistor or other heterostructure bipolar transistor.In addition, the invention is not limited to n-type transistors, it canalso be used for pnp transistors. In addition, the device is not limitedto silicon, and it can alternatively be used in germanium,germanium-silicon, III-V and SiC bipolar devices.

[0082] The specific dimensions and materials of the specific embodimentscan be varied, as will be obvious to those skilled in the art.

1. A bipolar transistor comprising a collector region (1) with a firstdoping type, a base region (2) with a second doping type, and an emitterregion (3) with the first doping type, a junction (4) being situatedbetween the emitter region (3) and the base region (2), and, viewed fromsaid junction (4), a depletion region (5) extending in the emitterregion (3), and, said emitter region (3) comprising a layer (6) of afirst semiconductor material and a layer (7) of a second semiconductormaterial, characterized in that the intrinsic carrier concentration ofthe second semiconductor material exceeds the intrinsic carrierconcentration of the first semiconductor material, the layer (7) of thesecond semiconductor material is situated outside the depletion region(5), and the second semiconductor material is doped such that Augerrecombination occurs.
 2. A bipolar transistor as claimed in claim 1,characterized in that the second semiconductor material has acomposition that is at least substantially constant over at least a part(8) of the layer (7).
 3. A bipolar transistor as claimed in claim 1 or2, characterized in that the first semiconductor material comprises atleast predominantly silicon, and the second semiconductor materialcomprises a composition of silicon and germanium.
 4. A bipolartransistor as claimed in claim 1, 2 or 3, characterized in that thelayer (7) has a doping with a doping concentration of at least 3×10²⁰cm⁻³.
 5. A bipolar transistor as claimed in claim 3, characterized inthat the percentage of germanium in the composition is below 30%.
 6. Abipolar transistor as claimed in claim 1, 3 or 4, characterized in thatthe layer (7) has a thickness (9) which, viewed perpendicularly from thejunction, is above 3 nm.
 7. A bipolar transistor as claimed in claim 1,characterized in that the emitter region (3) has an emitter contact(10), and the layer (7) with the second semiconductor material is atleast substantially adjacent to the emitter contact (10).
 8. Asemiconductor device (11) comprising a semiconductor body (12) of thefirst semiconductor material, and provided with a bipolar transistor asclaimed in any one of the preceding claims 1 through
 7. 9. A method ofmanufacturing a bipolar transistor comprising a collector region (1)with a first doping type, and a base region (2) with a second dopingtype on which an emitter region (3) with a first doping type is formed,the emitter region (3) comprising a layer (6) of a first semiconductormaterial and a layer (7) of a second semiconductor material,characterized in that the emitter region (3) is formed by epitaxiallyproviding a first layer (6) of the first semiconductor material, afterwhich a second layer (7) of the second semiconductor material issubsequently epitaxially provided and doped with a first doping typesuch that Auger recombination occurs, and the intrinsic carrierconcentration of the second semiconductor material exceeds the intrinsiccarrier concentration of the first semiconductor material.
 10. A methodas claimed in claim 9, characterized in that the composition of thesecond semiconductor material on the second layer (7) is at leastsubstantially constant.
 11. A method as claimed in claim 9 or 10,characterized in that the first semiconductor material providedcomprises at least substantially silicon, and the second semiconductormaterial provided comprises a composition of silicon and germanium. 12.A method as claimed in claim 9, 10 or 11, characterized in that thedoping provided in the second layer (7) has a doping concentration above3×10²⁰ cm⁻³.
 13. A method as claimed in claim 9, 10 or 11, characterizedin that the second layer (7) of the second semiconductor material isdoped in situ with a first doping type during the epitaxial growthprocess.
 14. A method as claimed in claim 9, 10, 11 or 12, characterizedin that an emitter contact (10) is formed on the emitter region (3) byproviding a polysilicon layer (16) with a first doping type on theemitter region (3), and the second layer (7) is doped by outdiffusion ofthe doping atoms from the polysilicon layer (16).
 15. A method asclaimed in claim 11, characterized in that the percentage of germaniumin the composition is chosen to be smaller than 30%.
 16. A method asclaimed in claim 9, characterized in that the thickness of the secondlayer (7) is chosen to be above 3 nm.
 17. A method of manufacturing asemiconductor device (11) comprising a semiconductor body (12) of thefirst semiconductor material, provided with a bipolar transistormanufactured in accordance with the method as claimed in claims 9through 16.